(1). Field of the Invention
This invention pertains to the field of dynamoelectric devices such as electric motors and generators that are used to convert energy in either electrical or mechanical form into the other. More particularly, this invention pertains to the use of a self-locking retaining member, frequently referred to as a top stick, positioned between stator poles to prevent windings from entering the air gap between the stator and rotor. The use of self locking retaining members increases the safety and reliability of dynamoelectric devices by preventing undesirable axial translation of retaining members that may occur during severe operating conditions of dynamoelectric devices.
(2). Description of the Related Art
There are numerous types of dynamoelectric devices in the prior art. A typical dynamoelectric device of the prior art in represented generally by the numeral 20 as shown in cross-section in FIG. 1. In general, the dynamoelectric device is comprised of a rotor 22 that is revolvable about an axis 24, a plurality of stator poles 26 positioned circumferentially about the rotor 22, and windings 28. A slot 30 extending in the direction of the axis 24 is formed between every two immediately adjacent stator poles 26, as is more clearly shown in detail in FIG. 2. The windings 28 consist of at least one electrically conductive coated wire wound within the slots 30 about one or more of the stator poles 26.
Typically, a slot liner 32 is positioned between the windings 28 and the stator poles 26 within the slots 30 to prevent the windings 28 from directly contacting the stator poles 26. The slot liners 32 are generally rectangular insulating sheets made of polymeric or fibrous material and have two opposite ends 33, the first end 33 positioned adjacent one of the two stator poles 26 that defines the slot 30 and the second end 33 positioned adjacent the other stator pole 26. Additionally, a liner cap 34 made of material similar to the slot liner 32, commonly referred to as a wedge, may extend from the first end 33 of the slot liner 32 to the second end 33, thereby covering the inner most surface of the windings 28. By "inner" and "inwardly", what is meant is, radially inward toward the axis 24 of the rotor 22.
A failure mode of such prior art devices arises when the windings 28 migrate from between stator poles 26 radially inward into the air gap 36 between the rotor 22 and stator poles 26, interfering with the moving rotor assembly. This failure occurs frequently in devices with large slot fills or large slot openings between stator poles wherein typical wire retention methods are insufficient. This is often a concern in switched reluctance motors that are subject to high winding temperatures or vibration loads. Recently, this failure mode has become an increasing concern when utilizing switched reluctance motors in safety critical applications such as in the automotive steering industry.
In those prior art devices having liner caps 34, they are generally radially non-rigid and therefore unable to prevent radial migration of the windings 28. One method utilized in prior art devices to prevent winding migration has been to varnish the windings, thereby preventing the wire passes that comprise the winding from moving independently of one another. This method has been shown to be beneficial in preventing winding migration but not at elevated winding temperatures. At elevated winding temperatures, the varnish strength is reduced and the varnish may therefore be unable to prevent winding migration. Additionally, the varnishing process typically has a considerably lengthy cycle time and high burden cost during production of dynamoelectric devices.
To reduce the cycle time and burden cost associated with the varnishing method, a similar method of preventing the wire passes that comprise the winding from moving independently of one another has been to utilize bondable wire coatings. This method, as is the case with the varnish method, is beneficial in preventing winding migration but not at elevated winding temperatures.
To prevent winding migration inherent to both the varnish and bondable wire coating methods at elevated temperatures, retaining members, commonly referred to as top sticks, have been developed in the prior art to provide a barrier between the windings 28 and the air gap 36 between the stator and rotor. This method is typically used as a secondary restraint in conjunction with other retention methods such as utilizing a bondable coating on the windings 28. Prior art retaining members are generally rectangular members that are slid axially into the slots between stator poles during assembly of the dynamoelectric devices. A typical prior art retaining member 38 is shown in FIGS. 3-5. The top stick retaining member 38 is shown in a plan view in FIG. 3 with its radially inner side shown. The retaining member is shown in cross section in FIG. 4. FIG. 5 is a partial view of the retaining member in one operative environment assembled in a dynamoelectric device between a pair of adjacent stator poles 26 and retaining a winding 28 between the poles.
The prior art retaining member 38 is formed of a generally rigid material that provides a barrier in the slot 30 between the windings 28 and the air gap 36. As can be seen in FIGS. 4 and 5, the prior art retaining member 38 has a radially inner surface 40 that faces inwardly toward the rotor and has a width slightly less than that of the slot 30 inwhich it is positioned. The inner surface 40 is often slightly curved, matching the radius of the inward most surfaces of the stator poles 26 so as to maintain a uniform air gap 36 between the rotor 22 and stator poles 26. The T-shaped cross section of the main body of the retaining member 38 as seen in FIG. 4 provides the main body with side rails 42 that are slightly farther apart than the narrowest portion of the width of the slot 30. The rails 42 engage with inner edges 43 of the pair of adjacent stator poles to hold the retaining member 38 radially in the slot. Thus, when an inward force is applied to the prior art retaining member, the side rails 42 prevent the prior art retaining member 38 from translating inward by engaging both the stator poles 26 that define the slot 30.
In addition to the main body, prior art retaining members 38 have also been provided with end stops 44 positioned at one axial end of the retaining member 38. The end stops 44 project outwardly beyond the width of the retaining member's inner surface 40 as seen in FIG. 4 and therefore are unable to fit within the slot 30 between adjacent stator poles 26. During installation into a dynamoelectric device, the end of the retaining member 38 axially opposite the end stops 44 is inserted axially into the slot 30. The configuration of the retaining member 38 allows it to slide axially between the inner edges 43 of the adjacent stator poles 26 until the end stops 44 engage the stator poles 26 or another axially rigid portion of the device. Thereafter, the end stops 44 prevent axial over insertion of the retaining member 38, thereby increasing the ease of their installation.
Retaining members are advantageous over other prior art solutions in that, when properly axially aligned, they effectively prevent excessive winding migration toward the gap 36 between the stator and rotor while maintaining a low burden cost during production. However, a disadvantage associated with such prior art retaining members has been an undesired axial movement of the retaining members as a result of severe three dimensional vibration and thermal expansion and contraction of the stator poles relative to the retaining member during the life of the dynamoelectric device. In prior art devices which utilize the varnish method in combination with retaining members 38, axial movement of the retaining members is limited slightly by the varnish if the retaining member is installed prior to the varnishing process. In prior art devices which utilize the bondable wire coating method, prior art retaining members 38 do not have any means other than friction against the inner edges 43 of the adjacent stator poles 26, windings 28, or against the liner cap 34 that prevents them from axially translating in a direction opposite to the direction in which they were inserted and tend to back-out in such severe situations. Additionally, the amount of friction is often minimized to ease assembly of the retaining member. The axial migration of the prior art retaining members can result in a portion of the windings entering the air gap between the rotor and stator or ultimately contacting the rotor assembly, thereby decreasing the performance or life of the device. Axial migration of the retaining members may also result in undesired interference of the retaining members with other rotating parts located on the rotor assembly.
The present invention overcomes the disadvantages associated with the use of prior art retaining members by providing a locking mechanism on the retaining members so as to prevent their undesired axial movement. The invention provides reliable operation of dynamoelectric devices without the need for costly varnishing and without adding additional components to prior art devices already utilizing retaining members.